Not to scale

In earth science, you often find yourself working and thinking over a range of different lengthscales and timescales – from micrometres to megametres*, from seconds to petaseconds**, or more. Because of this, making sure you’re clear on exactly what scale you’re meant to be thinking at today is an important part of a geologist’s mental toolkit, and in some of us it translates into a certain obsession with scale bars. For example, when folds kilometres across can – and do – resemble centimetre scale ones, it is nice to know exactly how big the photo or sketch you’re looking at actually is.

Folds the size of an outcrop? The size of a cliff? The size of a mountain? You decide!

This obsession with scale bars is mostly harmless, generally manifesting itself in stationary choices entirely motivated by how good they look in field photos.

I've yet to meet a geologist who doesn't think that these are awesome. From Steve Gough.

But it does give us a tendency to mutter about wrong, inappropriate and missing scales wherever we come across them (or, in the latter case, where we don’t – generally in students’ field notebooks). Of course, nowadays, Twitter allows those mutterings to be transmitted to a much larger audience.

Dear documentary makers: If you have a graphic in your show that is not to scale, please label “NOT TO SCALE”. That is all.

Erik’s tweet has got me wondering if I am as rigorous in this regard as I should be on this blog. Take, for example, this figure from my last post, which illustrates the elevation and subsidence of the Japanese coast over an earthquake cycle. These movements are measured in metres to tens of metres, so on this figure they have been clearly exaggerated; for reference, the width of this cross section is around 250 kilometres, and the crust on the Pacific plate is 10-15 km thick.

Click to see the post that features this figure

The problem here is not my selective application of a massive vertical exaggeration on this figure. I’m talking about the uplift and subsidence is terms of regional tectonics and seismology, so I needed to show the structure of the subduction zone. If, having done this, I kept to the same scale for the coastal movements, the topographic profiles would look like this.

No vertical exaggeration.

This may be more accurate, but it’s not particularly illuminating. However, not highlighting the vertical exaggeration is a potential problem. I, and other geologists reading this blog, can automatically compensate for the lack of proper scaling, but we can only do this because we already know that these motions are of the order of 10 metres, rather than 100 metres, or 1000. Without this prior knowledge, even if interested laypeople work out, without being told, that the scale is off, they may not know by how much – which may lead to confusion. This confusion could be avoided with a bit more rigour on my part.

Of course, compared to the offence that prompted Erik’s original tweet, I don’t think I’m in too much trouble.

@Allochthonous I was mostly irritated by a graphic in a NOVA on Toba that implied the magma chamber went down to the core-mantle boundary.

Comments (3)

I admit to being a bit of a scale pedant too. The one that drives me crazy is maps or photographs that have been unilaterally stretched, resulting in an anisotropic scale and things changing shape. It’s just a matter of being practical in sections, as in your example, but a map should have the same scale in x and y. Geophysicists seem especially fond of this atrocity.

I’m sure I’ve broken my own “scale?!” rule lots of times though. As you allude to, it’s so obvious to the author, but when something you’re only half familiar with lacks a scale, you spot it right away.

@Lab Lemming: Scale bar GPS is genius. I always thought it would be a great idea, and maybe not too hard, to build a scalebar into a digital camera. It could be rendered according to distance and focal length, based on some sort of rangefinder and the camera’s own data.

Good point about the lack of scale and vertical exaggeration notation in your cartoon of the Japan seismic cycle deformation. It didn’t bother me since I realized immediately that it was an exaggerated cartoon to illustrate the point, but then I am a geophysicist. Certainly better to explain this on a blog for the general public.